Great game, but a not enough new stuff.
One of the first new things that you will find is the updated weapons. This game involves traveling back forth between Light and Dark worlds, much like the system used by Legend of Zelda: A Link to the Past. However, this can be very frustrating. In MP2, you gain 3 new weapons. The Light Beam, Dark Beam, and Annilihator Beam( I believe it is called that.) These can be confusing and hard to switch between at times where you have to shoot something using the normal energy weapon, then immediately switch in order to defeat some Dark enemy.
Another thing that gets very frustrating involving the worlds is the loss of life. For every second you spend in the Dark world, out of reach of "crystals" that shed light into a small region, you lose life. This ups the difficulty level quite a bit, for if you don't work fast, you will end up dying, or getting yourself killed as soon as you enter a fight.
Another thing Nintendo added was the multiplayer mode. This doesn't seem up to par with the rest of the game. For one, it is extremely easy to kill you opponent. Simply lock on and launch about 5 missiles. The lock-on system is what really makes this easy. It would have been good for Retro to take away the lock on system for the multiplayer. This, however, may have made it too hard, considering the fact that moving and aiming at the same time can be difficult. Also, any weapons upgrades don't last for more than a minute. This doesn't work well, for if you are playing with only 2 or 3 people on a larger map, finding someone to kill within that minute can be hard. A better option would be to let you keep the upgrade until you die, or for some longer time, depending on the length of the game. Maybe 1/4 the total game time, perhaps?
All in all, this game is good, it just isn't quite innovative enough. That would be great, but lately that's the biggest word in Nintendo's dictionary.